Direct steam generation of boiler blowdown

ABSTRACT

Systems and methods generate steam from produced water by passing the produced water through first and second steam generators coupled together. The first steam generator produces wet steam in which a liquid effluent with impurities of the produced water passes to the second steam generator. The second steam generator combusts fuel and oxidant in direct contact with the liquid effluent. The first and second steam generators limit fouling and waste while providing a combined steam output that may include combustion products from only the second steam generator.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a non-provisional application which claims benefit under 35 USC §119(e) to U.S. Provisional Application Ser. No. 61/717,676 filed Oct. 24, 2012, entitled “DIRECT STEAM GENERATION OF BOILER BLOWDOWN,” which is incorporated herein in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

None

FIELD OF THE INVENTION

Embodiments of the invention relate to methods and systems of handling blowdown from initial steam generation with additional direct steam generation.

BACKGROUND OF THE INVENTION

Enhanced oil recovery processes employ thermal methods to improve recovery of heavy oils from subsurface reservoirs. For example, injection of steam into heavy oil bearing formations heats the oil in the reservoir, which reduces the viscosity of the oil and allows the oil to flow to a collection well. A mixture of the oil and produced water that flows to the collection well is recovered to the surface where the oil is separated from the water.

For economic and environmental reasons, these operations recycle the water used in the steam injection by treating the produced water and directing treated feedwater to a steam generator or boiler. Several treatment processes remove constituents which form harmful deposits in the boiler or steam generator but without even further costs do not remove all dissolved solids. Blowdown in steam generation thus includes impurities and can include about a quarter of the feedwater input for a once-through type steam generator.

The blowdown presents a problem regardless of application requiring the steam. The impurities can limit reuse as feedwater without further expensive treatment or inefficient concentrating of the impurities. If not reused, the blowdown represents a significant and unacceptable water portion for disposal.

Therefore, a need exists for systems and methods to limit production of undesirable waste streams during steam generation.

SUMMARY OF THE INVENTION

In one embodiment, a method of generating steam includes injecting water into an initial steam generator to produce steam and a liquid effluent containing a remainder of the water with impurities. Injecting the liquid effluent into a direct steam generator vaporizes at least part of the liquid effluent upon contact with combustion products. A resulting gas phase including additional steam and carbon dioxide outputs from the direct steam generator separate from a non-gas phase waste stream.

For one embodiment, a system for generating steam includes an initial steam generator coupled to receive water and having a first output for steam and a second output for liquid effluent containing a remainder of the water with impurities. A direct steam generator couples to receive the liquid effluent for at least partial vaporization upon contact with combustion products. The direct steam generator includes a third output for a resulting gas phase including additional steam and carbon dioxide and a fourth output for a non-gas phase waste stream.

A method of generating steam includes recovering production fluid from a reservoir and separating the production fluid into an oil stream and a water stream. Generating steam from the water stream includes partial vaporization of the water stream while isolated from fluid communication with combustion products and then further vaporization of the water stream while in fluid communication with combustion products. The method further includes injecting the steam into the reservoir for a steam assisted gravity drainage operation.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the present invention and benefits thereof may be acquired by referring to the follow description taken in conjunction with the accompanying drawings.

FIG. 1 depicts a schematic of a system including initial and direct steam generators coupled together for a heavy oil extraction process, according to one embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the invention relate to systems and methods of generating steam from produced water by passing the produced water through first and second steam generators coupled together. The first steam generator produces wet steam in which a liquid effluent with impurities of the produced water passes to the second steam generator. The second steam generator combusts fuel and oxidant in direct contact with the liquid effluent. The first and second steam generators limit fouling and waste while providing a combined steam output that may include combustion products from only the second steam generator.

FIG. 1 depicts a production well 100, a separator 102, a treatment facility 108, an initial steam generator 112, a direct steam generator 118 and an injection well 114 used in conjunction with heavy oil or bitumen extraction from a reservoir. During the extraction, steam injected into the reservoir through the injection well 114 increases the mobility of the sought after oil within the reservoir. In an exemplary embodiment, the injection well 114 and the production well 100 from a well pair for a steam assisted gravity drainage (SAGD) operation.

In operation, a liquid mixture of oil and water recovered through the production well 100 enters the separator 102, which outputs an oil stream 104 divided from a water stream 106. The water stream 106 passes through the treatment facility 108 that removes contaminants in the water stream 106 to a level acceptable for the initial steam generator 112. The treatment facility 108 may include de-oiling, such as by addition of a de-oiling polymer, warm lime softening, filtering and/or weak acid cation exchanging.

Treated water 110 then exits the treatment facility and passes to the initial steam generator 112. In some embodiments, one or more once through steam generators (OTSG) provide the initial steam generator 112 such that water quality needed by the initial steam generator 112 is reduced compared to conventional drum boilers employing more expensive pretreatment schemes. The initial steam generator 112 further may produce a steam quality between 70 percent and 85 percent with such steam exiting through a first output to the injection well 114.

A remainder of the treated water 110 thus not converted into the steam goes to a liquid effluent 116 exiting the initial steam generator 112 through a second output coupled to the direct steam generator 118. The liquid effluent 116 furthermore contains impurities that were not removed by the treatment facility 108. The impurities in the liquid effluent 116 may make the liquid effluent 116 unsuitable for recycle through the initial steam generator 112.

The liquid effluent 116 however contains enough water content to make disposal undesirable. Replacement of the water not being recycled presents a problem in many areas with limited makeup water availability. In addition, limitations on volumes sent to waste disposal wells make additional concentrating of the liquid effluent 116 desirable.

The direct steam generator 118 enables producing additional steam from the liquid effluent 116 since not restrained by as stringent requirements on water quality as the initial steam generator 112. While the direct steam generator 118 may accept even untreated water, use in combination with the initial steam generator 112 may enable keeping carbon dioxide concentration injected into the reservoir below a certain threshold if desired to control carbon dioxide concentration based on potential for the carbon dioxide to impact recovery. The direct steam generator 118 may also provide solutions for handling the liquid effluent 116 from the initial steam generator 112 preexisting at some facilities.

One example of the direct steam generator 118 utilizes oxy-combustion by burning fuel, such as natural gas 120, and an oxidant, such as oxygen 122, in a pressurized chamber with the liquid effluent 116 injected into the chamber for chamber cooling upon vaporization into the steam. Unlike generating the steam in the initial steam generator 112 where water vaporization may occur while isolated from fluid communication with combustion products, water vaporization occurs while in fluid communication with combustion products using the direct steam generator 118. Products of the direct steam generator 118 include the steam, both from the combustion of the natural gas and the vaporization of the injected liquid effluent 116, and carbon dioxide from the combustion of the natural gas.

A resulting gas phase that exits the direct steam generator 118 through a third output contains about 80 to 95 weight percent steam along with carbonaceous combustion products, such as carbon dioxide. In some embodiments, the steam exiting the initial steam generator 112 through the first output combines with the gas phase exiting the direct steam generator 118 through the third output to form a gas mixture prior to being conveyed into the injection well 114. The gas mixture that is introduced into the injection well 114 may therefore include steam with less than 5 percent carbon dioxide by weight or between 2 and 4 percent carbon dioxide by weight.

For some embodiments, the direct steam generator 118 superheats the steam exiting through the third output. This superheating prevents condensation of the gas mixture prior to introduction into the injection well 114. While not desired, the condensation may otherwise occur since the steam from the initial steam generator 112 may cool as conveyed from a central processing facility to a wellpad.

With respect to impurities in the effluent stream 116 from the initial steam generator 112, the impurities may include one or more of NaCl, Ca, Mg, Na, K, Fe⁺³, Mn⁺², Ba⁺², Sr⁺², SO₄, Cl, F, NO₃, HCO₃, CO₃, PO₄, SiO₂ and combustible compounds, such as tar, gas, oil, dioxins, nitrogen and organometallic compounds. Inside the direct steam generator 118, the combustible compounds combust and form part of the combustion products. Excess oxygen 122 supplied to the direct steam generator 118 may ensure that the combustible compounds are burned.

Some of the impurities, such as Na+ and Cl−, form solid particles (i.e., NaCl crystals) if the direct steam generator 118 is operated for complete vaporization of all the water inside the direct steam generator 118. A variety of different phase separation techniques can remove the solid particles from the gas phase exiting the direct steam generator 118. For example, the direct steam generator 118 may utilize a cyclone or filters to separate the solid particles into a non-gas phase waste stream 124 the exits the direct steam generator 118 through a fourth output.

The non-gas phase waste stream 124 may contain the solid particles with no liquid or substantially no liquid. All water may thus get recycled in some embodiments. The solid particles enable disposal of waste in a landfill.

For some embodiments, a brine or slurry forms the non-gas phase waste stream 124 due to incomplete vaporization of the liquid effluent 116 supplied in excess of a saturation amount for the direct steam generator 118. Vaporization energy generated by the combustion of the natural gas 120 and the oxygen 122 determines the saturation amount. The impurities from the effluent stream 116 thereby concentrate in the non-gas phase waste stream 124, which may have sufficient low water content to be acceptable for injection into a disposal well. Further treatment of the non-gas phase waste stream 124 may also remove the impurities as solid waste and produce water suitable for recycle to the initial steam generator 112 or the direct steam generator 118.

In some embodiments, the non-gas phase waste stream 124 as shown enables rejection and disposal of all the impurities remaining after steam generation without any recycle thereof. Past configurations require recycling a portion of aqueous blowdown streams to meet regulatory requirements resulting in some of the impurities being added back into feed streams for the steam generators. These impurities that get recycled contribute to making quality of the blowdown streams worse due to gradual buildup and also to increasing fouling of the steam generators that thereby need more frequent expensive cleaning cycles, which hinder production.

The preferred embodiments of the invention have been disclosed and illustrated. However, the invention is intended to be as broad as defined in the claims below. Those skilled in the art may be able to study the preferred embodiments and identify other ways to practice the invention that are not exactly as described herein. It is the intent of the inventors that variations and equivalents of the invention are within the scope of the claims below and the description, abstract and drawings are not to be used to limit the scope of the invention. 

1. A method of generating steam, comprising: injecting water into an initial steam generator to produce steam and a liquid effluent containing a remainder of the water with impurities; and injecting the liquid effluent into a direct steam generator to vaporize at least part of the liquid effluent upon contact with combustion products such that a resulting gas phase including additional steam and carbon dioxide is output separate from a non-gas phase waste stream.
 2. The method of claim 1, wherein the liquid effluent vaporizes in the direct steam generator leaving the waste stream formed of solid particles.
 3. The method of claim 1, wherein the waste stream includes liquid brine formed by incomplete vaporization of the liquid effluent supplied in excess of a saturation amount for the direct steam generator.
 4. The method of claim 1, wherein the impurities include combustible compounds combusted by excess oxidant supplied to the direct steam generator.
 5. The method of claim 1, wherein the steam from both the initial and direct steam generators is injected into a well for a steam assisted gravity drainage operation.
 6. The method of claim 1, wherein the steam from the direct steam generator is superheated.
 7. The method of claim 1, wherein the steam from both the initial and direct steam generators is injected into a well and the steam from the direct steam generator is superheated to a temperature that prevents condensation prior to injection into the well.
 8. The method of claim 1, wherein the steam from the initial steam generator and the gas phase from the direct steam generator are combined into a mixture that is injected into a well and includes steam with less than 5 percent carbon dioxide by weight.
 9. The method of claim 1, wherein the steam from the initial steam generator and the gas phase from the direct steam generator are combined into a mixture that is injected into a well and includes steam with between 2 and 4 percent carbon dioxide by weight.
 10. The method of claim 1, wherein the initial steam generator includes a once through steam generator that produces a steam quality between 70 percent and 85 percent with the remainder of the water going to the liquid effluent.
 11. A system for generating steam, comprising: an initial steam generator coupled to receive water and having a first output for steam and a second output for liquid effluent containing a remainder of the water with impurities; and a direct steam generator coupled to receive the liquid effluent for at least partial vaporization upon contact with combustion products and having a third output for a resulting gas phase including additional steam and carbon dioxide and a fourth output for a non-gas phase waste stream.
 12. The system of claim 11, wherein the direct steam generator vaporizes the liquid effluent leaving the waste stream formed of solid particles.
 13. The system of claim 11, wherein the direct steam generator produces the waste stream of liquid brine formed by incomplete vaporization of the liquid effluent.
 14. The system of claim 11, wherein the impurities include combustible compounds combusted by excess oxidant supplied to the direct steam generator.
 15. The system of claim 11, further comprising a well for a steam assisted gravity drainage operation that is coupled to both the initial and direct steam generators for steam injection.
 16. The system of claim 11, wherein the direct steam generator produces the steam that is superheated.
 17. The system of claim 11, further comprising a well coupled to the first and third outputs such that a mixture injected into the well includes steam with less than 5 percent carbon dioxide by weight.
 18. The system of claim 11, further comprising a well coupled to the first and third outputs such that a mixture injected into the well includes steam with 2 to 4 percent carbon dioxide by weight.
 19. The system of claim 11, wherein the initial steam generator includes a once through steam generator that produces a steam quality between 70 percent and 85 percent with the remainder of the water going to the liquid effluent.
 20. A method of generating steam, comprising: recovering production fluid from a reservoir; separating the production fluid into an oil stream and a water stream; generating steam from the water stream by partial vaporization of the water stream while isolated from fluid communication with combustion products and then further vaporization of the water stream while in fluid communication with combustion products; and injecting the steam into the reservoir for a steam assisted gravity drainage operation. 